If You Have to... You Have to...
You have to figure out what is important and what to ignore. Check the syllabus
for the course. Check the course description. Check your notes for underlined
points that describe major themes, structure, concepts. You need to understand
those. Cramming leaves no time for niceties. You need the main points and
nothing else.

You Need a Plan
Now figure out how much time you have. Be conservative. It's no good making
a plan that you don't carry out. Allow for sleep, eating, breaks etc. How
many hours do you have?

Work out a cramming schedule. Spend 30-35% of the remaining time
going over the material that you deem the most important. Spend
the remaining 65-70% of your time in repeating that material to
yourself until you know it. Here's how.

Go over the material you have selected for your study effort.
Read all the notes, paying attention to underlining or any emphasis
that has been recorded. Now use your notes (or the textbook if
you must) to create an outline of the material you are studying.
Write down the broad concepts, use itemized lists for the details,
include any examples taken from lectures, and in particular, any
information highlighted in the notes as being emphasized by the
instructor (this is why attending lectures/class is important).

Now go over the material again, formulating questions that might
be asked on an exam or test, and answering those questions using
the notes. Record the questions in your outline, along with point
form answers.

And repeat.

And repeat.

And repeat.

At some point, when you believe you have a decent grasp of the
material, start using the outline you created. Essentially you
want to memorize the outline, and be able to re-write it from memory.
That tells you it should be condensed, with no fluff, just all
stuff. That memorized outline is going to be your salvation.

SLEEP! If you don't get enough sleep your brain won't retain all
the information that you have crammed in there.

If You Don't Have Notes
If you don't have notes, you need to essentially create some for yourself.
Since you don't have a record of what the professor considers most important,
you will have to figure it out on your own. Go through the textbook, paying
special attention to the introduction, the first paragraph of each chapter,
chapter summaries and any questions or study notes at the end of each chapter
(sometimes these are included in the body of the chapter as breakout boxes).

The introduction should give you a good clue as to where to focus.
While going over the material and making notes (as described above)
pay particular attention to examples in the text. You need to understand
them. If you had taken notes in class, the prof would have presented
examples to aid in understanding. You will have to do your best
on your own. And that's it. Carry on as above, except that you
will have to wade through more material. It is extra important
for you to create an outline. You don't want to go through the
text more than once (after you have identified what material you
will be focusing on).

Don't Cover Everything
Avoid the temptation to go over everything in a shallow manner. You will then
master nothing. Remember, at this point your aim is a pass. Using these cramming
strategies, you have already identified the most significant material. And you
have focused heavily on it. The questions with the most marks and most of the
large mark questions should come from material you studied. Don't do yourself
the disservice of just skimming the important information so that you have time
to also skim the least important.

Avoid Cramming... Why?Your anxiety level will go up
You will lose sleep and eat poorly because of this
You will get sick more easily because of this
You will miss the exam because of this
You will take the much harder essay make-up exam because of this
You will fail the exam.
Seriously, at a minimum you will do worse on the exam than you would have otherwise.
Guaranteed.